The present invention relates to a method and a device for measuring and calculating refraction errors of the eye.
The refraction error of an eye is represented by the refracting power of a lens which would compensate for the refraction irregularity. More specifically, the error is represented by a spherical power (S), a cylindrical power (C) and a cylinder axis (.theta.) of the cylindrical surface. There is a mathematical relation between the refracting power (R) and the abovementioned values as follows: PS EQU R=S+C.multidot.sin.sup.2 (.theta.-.alpha.),
wherein .alpha. represents a measured meridian angle. Thus, it is possible to obtain refraction errors (S, C, .theta.) from at least three data points measured for the meridian angle and the refraction power R.
In a prior art eye refraction error measuring apparatus there is a difficulty in obtaining a signal having a sufficiently high S/N (signal/noise) ratio because the reflected light from the retina of the eye is extremely weak and it includes a considerable amount of noise components such as light reflected from the cornea and the like. There is also a problem in obtaining measured results of high reliability.
Japanese Published Patent Application No. 2519/79 discloses an apparatus which attempts to overcome such difficulties. In this apparatus, the meridian angle .alpha. to be measured can be varied within a wide range, measured results of a signal representing a refraction value corresponding to an optimum focal point at each meridian angle are simulated by a sine wave, and refraction errors are calculated from the sine wave. In order to assure accurate calculation results, the difference between the signal and the sine wave is analyzed prior to the calculation of the refraction errors. If the difference is found to be excessive, this fact is indicated to the operator, and when the difference is sufficiently small, the calculated refraction errors are printed.
However, when it is desired to obtain refraction values for several meridian angles, the measuring system of the apparatus must be rotated for each different angle thus requiring considerable time for obtaining the necessary data (30 sec. is required for collecting data and processing the data). During the measurement, the eye must be kept at rest and focussed at infinity. When the measuring time becomes long, such a condition is hardly attainable resulting in deviations in the measured data and making it difficult to obtain reliable results. Furthermore, with this apparatus, there is no assurance of obtaining reliable refraction values R.sub.i (i=1, 2, 3, . . . ) from the data measured at meridian angles .alpha..sub.i thus making it difficult to obtain reliable results.
The eye refraction error measuring method and device according to the invention are intended to overcome these difficulties.